Energy is an important material basis for survival and development of human society. With the development of economy and society, demand for energy is increasing. Today, due to global warming and ecological environment deteriorating, lithium-ion batteries as a green energy has gained growing concern.
More attention has been paid to lithium iron phosphate due to its higher voltage platform (3.4-3.5V), higher theoretical capacity (170 mA·h/g), good recycle performance (LiFePO4 can be recycled more than 2000 times under optimized synthesis conditions), good stability, good high temperature performance, good safety performance, simple synthesis process, environmentally friendly, nontoxicity, abundant raw material and other characteristics.
However, the uniqueness of the structure of the lithium iron phosphate blocks conduction of electrons, so that electronic conductivity of lithium iron phosphate is very low (10−9 S·cm−1), resulting in poor electrochemical performance. People often coat the surface of lithium iron phosphate particles with a layer of conductive carbon to increase its conductivity. Such as Chinese Patent (CN101483236) discloses a method for preparing lithium iron phosphate/carbon composite as cathode material of lithium ion battery, which includes mixing iron oxide hydroxide, lithium salt, and phosphonium salts in stoichiometric ratio; adding an appropriate amount of carbon source and liquid ball milling media; milling; reacting the mixture after drying at a certain temperature; and finally obtaining the lithium ferrous phosphate/carbon composite.
Graphene is a new material from carbon family which becomes very popular in recent years, with thickness of only 0.335 nm, and has a unique electronic structure and electrical properties. In the energy band structure of graphene, the valence band and the conduction band intersect at six vertices of the Fermi level, and in this sense, graphene is a material without energy gap, showing metallicity. In graphene of single layer, each carbon atom contributes a non-bonded electron, and these electrons can move freely in the crystal, giving graphene very good electrical conductivity. Typical electronic conduction velocity in graphene reaches 1/300 of the speed of light, much more than the conduction velocity of electron in a general semiconductor. Therefore, it is possible to combine excellent electrical properties of graphene with special electrochemical performance of lithium iron phosphate properly and cleverly to develop graphene-modified lithium iron phosphate cathode composite material. For example Chinese Patent (CN101752561A) discloses a graphene-modified lithium iron phosphate cathode active material and method for preparing thereof, as well as a lithium ion secondary battery on basis of the cathode active material, and the method includes dispersing graphene or graphene oxide and lithium iron phosphate in an aqueous solution; mixing well by stirring and ultrasonic, subsequently drying to obtain graphene or graphene oxide complexed lithium iron phosphate material, and then high temperature annealing to finally obtaining lithium iron phosphate cathode active material which only is subjected to simple graphene modification.